Estática, Unidades, Cálculos e Solução de Problemas

Objetivos de Hoje:

Os estudantes devem ser capazes de:

a) Identificar o que é Mecânica/Estática

b) Trabalhar com dois tipos de Unidades

c) Arrendodamento.

d) Aplicar estratégias para solução. Atividades de Aula:

• O que é mecânica

• Sistema de Unidades

• Cálculo numérico

• Estratégias de Solução

PERGUNTA

1. O objetivo da mecânica é entender o que ocorre com um corpo quando______ é aplicado a ele.

A) campo magnético B) calor C) forças

D) neutrons E) lasers

2. ________________ CONTINUA SENDO A BASE DAS CIENCIAS DE ENGENHARIA DE HOJE.

A) Mecânica Newtoniana B) Mecânica relativista

C) Mecânica Euclidiana C) Mecânica Grega

O QUE É MECÃNICA??

• Either the body or the forces could be large or small.

• Study of what happens to a “thing” (the technical name is “body”) when FORCES are applied to it.

BRANCHES OF MECHANICS

S ta tic s D yn am ics

R ig id B od ies(Th in g s th a t d o n o t ch an g e sh ap e)

D efo rm ab le B od ies(Th in g s th a t d o ch an g e sh ap e)

In com p ress ib le C om p ress ib le

F lu id s

M ech an icsTyp e t it le h ere

What may happen if static's is not applied properly?

SYSTEMS OF UNITS (Section 1.3)

• Four fundamental physical quantities.

• Length, mass, time, force.

• One equation relates them, F = m * a

• We use this equation to develop systems of units

• Units are arbitrary names we give to the physical quantities.

UNIT SYSTEMS

• Define 3 of the units and call them the base units.

•Derive the 4th unit (called the derived unit) using F = m * a.

• We will work with two unit systems in static’s: SI and US

Customary.

Table 1-1 in the textbook summarizes these unit systems.

COMMON CONVERSION FACTORS

• 1 ft = 0.3048 m

• 1 lb = 4.4482 N

• 1 slug = 14.5938 kg

• Example: Convert a torque value of 47 in • lb into SI units.

• Answer is 5.31026116 N • m??

• Work problems in the units given

unless otherwise instructed!!

RULES FOR USING SI SYMBOLS (Section 1.4)

• No Plurals (e.g., m = 5 kg not kgs )

• Separate Units with a • (e.g., meter second = m • s )

• Most symbols are in lowercase ( some exception are N,

Pa, M and G)

• Exponential powers apply to units , e.g., cm2 = cm • cm

• Other rules are given in your textbook

NUMERICAL CALCULATIONS (Section 1.5)

• Must have dimensional “homogeneity.” Dimensions have to be the same on both sides of the equal sign, (e.g. distance = speed time.)

• Be consistent when rounding off.

- greater than 5, round up (3528 3530)

- smaller than 5, round down (0.03521 0.0352)

- equal to 5, see your textbook.

• Use an appropriate number of significant figures (3 for

answer, at least 4 for intermediate calculations). Why?

SAMPLE CONCEPT QUIZ

1. Evaluate the situation, in which mass(kg), force (N), and length(m) are the base units and recommend a solution.

A) A new system of units will have to be formulated

B) Only the unit of time have to be changed from second to

something else

C) No changes are required.

D) The above situation is not feasible

SAMPLE CONCEPT QUIZ (continued)

2. Give the most appropriate reason for using three significant figures in reporting results of typical engineering calculations.

A) Historically slide rules could not handle more than three significant figures.

B) Three significant figures gives better than one-percent accuracy.

C) Telephone systems designed by engineers have area codes consisting of three figures.

D) Most of the original data used in engineering calculations do not have accuracy better than one percent

PROBLEM SOLVING STRATEGY: IPE, A 3 Step Approach

1. Interpret: Read carefully and determine what is given and what is to be found/ delivered. Ask, if not clear. If necessary, make assumptions and indicate them.

2. Plan: Think about major steps (or a road map) that you will take to solve a given problem. Think of alternative/creative solutions and choose the best one.

3. Execute: Carry out your steps. Use appropriate diagrams and equations. Estimate your answers. Avoid simple calculation mistakes. Reflect on / revise your work.

SAMPLE ATTENTION QUIZ

2. In three step IPE approach to problem solving, what does P stand for

A) Position B) PlanC) Problem

D) Practical E) Possible

1. For a static’s problem your calculations show the final answer as 12345.6 N. What will you write as your final answer?

A) 12345.6 N B) 12.3456 kN C) 12 kN

D) 12.3 kN E) 123 kN